How to Use string.Format in C# for Formatting Strings

Use string.Format in C# to create formatted strings by placing numbered placeholders like {0} in a string and passing values to replace them. It returns a new string with the placeholders replaced by the provided values in order.

Syntax

The basic syntax of string.Format is:

• string.Format(formatString, arg0, arg1, ...)
• formatString contains text with placeholders like {0}, {1}, etc.
• arg0, arg1, ... are the values to insert into the placeholders.

Each placeholder number corresponds to the argument index.

csharp

string.Format("Hello, {0}! You have {1} new messages.", "Alice", 5);

Example

This example shows how to use string.Format to insert a name and a number into a greeting message.

csharp

using System;

class Program
{
    static void Main()
    {
        string name = "Alice";
        int messageCount = 5;
        string result = string.Format("Hello, {0}! You have {1} new messages.", name, messageCount);
        Console.WriteLine(result);
    }
}

Output

Hello, Alice! You have 5 new messages.

Common Pitfalls

Common mistakes when using string.Format include:

• Using placeholder numbers that don't match the number of arguments, causing runtime errors.
• Forgetting to escape braces { or } if you want to show them literally by doubling them {{ or }}.
• Passing arguments in the wrong order, which leads to incorrect output.

csharp

using System;

class Program
{
    static void Main()
    {
        // Wrong: Missing argument for {1}
        // string result = string.Format("{0} scored {1} points.", "Bob"); // Throws exception

        // Correct: Provide both arguments
        string correctResult = string.Format("{0} scored {1} points.", "Bob", 42);
        Console.WriteLine(correctResult);

        // Showing literal braces
        string braces = string.Format("Use double braces to show {{ and }}.");
        Console.WriteLine(braces);
    }
}

Output

Bob scored 42 points.
Use double braces to show { and }.

Quick Reference

Here is a quick reference for string.Format placeholders and usage:

Placeholder	Description	Example
{0}	First argument	string.Format("{0}", "Hello") → "Hello"
{1}	Second argument	string.Format("{1}", "A", "B") → "B"
{{	Literal open brace	string.Format("{{{{") → "{"
}}	Literal close brace	string.Format("}}}}") → "}"
{0:N2}	Format number with 2 decimals	string.Format("{0:N2}", 3.14159) → "3.14"

Key Takeaways

Use string.Format with numbered placeholders like {0}, {1} to insert values into strings.

Make sure the number of placeholders matches the number of arguments to avoid errors.

Double braces {{ and }} show literal braces in the output string.

You can format numbers and dates inside placeholders using format specifiers.

string.Format returns a new formatted string without changing the original.